Generally, a supercharger is known in which a turbine wheel is rotated by exhaust gas from an engine so as to rotate an impeller via a turbine shaft constructed integrally with the turbine wheel, whereby sucked air (hereinafter, referred to as “intake air”) is compressed and sent to cylinders of a cylinder head. In the supercharger, temperature of a turbine housing in which the turbine wheel is housed is made very high by the heat of exhaust gas, therefore it is necessary to protect the other components of the engine and peripherals around the supercharger from radiation heat of the turbine housing. Especially, in an engine for a ship, for preventing ignition of combustible parts and oil by radiation heat so as to prevent certainly the fire in the ship, an art is required for reducing the radiation heat from the turbine housing.
Then, conventionally, an art is known in which a cooling jacket is formed in the turbine housing and coolant such as fresh water with low temperature is circulated in the cooling jacket so as to cool compulsorily the turbine housing, thereby reducing the radiation heat from the turbine housing to the circumference of the supercharger (for example, see the Patent Literature 1). An art is also known in which the turbine housing is covered by heat insulating material such as asbestos, lagging material in which the heat insulating material is confined, or a heat shield body such as a masking shield formed by enclosing the heat insulating material between metal plates, thereby reducing the radiation heat from the turbine housing to the circumference of the supercharger (for example, see the Patent Literature 2).
However, in the former art, though the heat of the turbine housing is absorbed by the coolant flowing in the cooling jacket and the temperature of the turbine housing is reduced so as to reduce the radiation heat from the turbine housing is reduced, heat loss of the exhaust gas with high temperature and high pressure flowing in a turbine chamber inside the turbine housing is increased so as to reduce the turbine efficiency, and the temperature of the coolant absorbing much heat is raised so that cooling efficiency in the case of cooling the engine or the like with the coolant is reduced. In the latter art, in the case that the supercharger is driven for long time, heat is accumulated by transmission and radiation of heat from the turbine housing so that the temperature of the turbine housing is raised remarkably and heat transmission from the turbine housing to the outside cannot be suppressed enough, whereby the outer surface of the heat shield body is heated and the radiation heat to the circumference of the supercharger is increased remarkably.
For suppressing the heat loss of the exhaust gas, the rise of temperature of the coolant, accumulation of heat and the like, it is conceivable to provide a cooling structure including an inner heat insulating part constructed by an air layer and an outer low temperature part covering, the inner heat insulating part around the turbine housing. According to the cooling structure, by interposing the inner heat insulating part, the turbine housing is prevented from touching directly the outer low temperature part, whereby the raise of temperature of the coolant in the outer low temperature part is reduced suitably while preventing excessive absorption of heat from the exhaust gas in the turbine chamber. Furthermore, the outer low temperature part absorbs efficiently heat transmitted from the turbine housing, thereby preventing accumulation of heat.    Patent Literature 1: the Japanese Utility Model Laid Open Gazette Hei. 4-76932    Patent Literature 2: the Japanese Utility Model Laid Open Gazette Hei. 6-73337